Kind of a loaded question, but I think rogues are one of those classes that are hard to balance, in the sense that either they are extremely strong or extremely weak. I don't have enough experience to back this feeling up however.

Rouges. 100%.
Nothing to do with subjective balance. I just really dont like the mechanic and play-style behind the class in PVP. i find it to be a very wonky match up against any class.
Ferals are pretty bad to, but not to the same extent.

I think it would be a great counter to fotm re-rollers to allow your arena team to ban one class spec you don't want to face as the opposing team. Would have seen less invincible BM hunters, arms warriors, and resto shamans the first month of MoP.

On topic I would have to say monks. This game is old enough to ask players to learn how to play against a completely new class yet again, nevermind the consistent change in talents, spells and mechanics every expansion for the remaining 10 classes.

If I couldn't choose monks I would probably choose warriors. Not because I particularly dislike warriors, but their design in general have always been quite floppy. Either they are this unstoppable juggernaught that are able to tank wayyy to much damage, with incredible mobility and able to deal humongous damage when they connect (and they will connect), especially since previously damage taken=rage, and if not warriors were just dealing too low damage, too easy to kite, and easily killed in a stun.

Since season 7-8 I've always felt that DKs seem more natural and well-round designed in PvP despite being a much younger class than warriors.